The background is described with respect to LTE (Long Term Evolution). The skilled person in the art will however realize that the principles of the invention may be applied in other radio communication systems, particularly in communication systems that rely on scheduled data transmissions.
The downlink transmission of the LTE, or E-UTRAN radio access, is based on Orthogonal Frequency Division Multiplex (OFDM). The basic LTE downlink physical resource can thus be seen as a time-frequency grid as illustrated in FIG. 1, where each resource element (RE) corresponds to one OFDM subcarrier during one OFDM symbol interval. The dark shadowed resource elements form a resource block. LTE uses OFDM in the downlink and DFT-spread OFDM in the uplink.
In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms, as depicted in FIG. 2.
Furthermore, the resource allocation in LTE is typically described in terms of resource blocks (RB), where a resource block corresponds to one slot (0.5 ms) in the time domain and 12 contiguous subcarriers in the frequency domain. A pair of two adjacent resource blocks in time direction (1.0 ms) is known as a resource block pair. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
The notion of virtual resource blocks (VRB) and physical resource blocks (PRB) has been introduced in LTE. The actual resource allocation to a user equipment (UE) is made in terms of VRB pairs. There are two types of resource allocations, localized and distributed. In the localized resource allocation, a VRB pair is directly mapped to a PRB pair, hence two consecutive and localized VRB are also placed as consecutive PRBs in the frequency domain. On the other hand, the distributed VRBs are not mapped to consecutive PRBs in the frequency domain; thereby providing frequency diversity for data channel transmitted using these distributed VRBs.
Downlink transmissions are dynamically scheduled, i.e., in each subframe the base station transmits control information about to which terminals data is transmitted and upon which resource blocks the data is transmitted, in the current downlink subframe. This control signaling is typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe and the number n=1, 2, 3 or 4 is known as the Control Format Indicator (CFI). The downlink subframe also contains common reference symbols (CRS), which are known to the receiver and used for coherent demodulation of e.g. the control information. A downlink system with CFI=3 OFDM symbols as control is illustrated in FIG. 3.
Release 10 of the LTE specifications from the 3rd Generation Partnership Project (3GPP) supports component carrier bandwidths up to 20 MHz. However, in order to meet the International Mobile Telecommunications Advanced (IMT-Advanced) requirements for very high data rates, the concept of carrier aggregation has been introduced to support bandwidths larger than 20 MHz. The carrier aggregation concept is illustrated in FIG. 4, where five component carriers are illustrated, with respective bandwidths of f1, f2, f3, f4 and f5. In the example of FIG. 4, the total bandwidth available to a mobile terminal is the sum of the bandwidths of the cells. In the following each component carrier is named as a cell.
A UE may be configured with a subset of the cells offered by the network and the number of aggregated cells configured for one UE can change dynamically through time based on for example UE traffic demand, type of service used by the UE, system load etc. A cell which a UE is configured to use is referred to as a serving cell for that UE. A UE has one primary serving cell (called PCell) which also handles some signaling and zero or more secondary serving cells (called SCells). The term “serving cell” includes both the PCell and SCells. It is UE specific which cell is defined as the PCell.
Aside from that the concept of configuration of cells has been introduced the concept of activation has been introduced for SCells. Cells may be configured or de-configured using Radio Resource Control (RRC) signaling according to 3GPP TS 36.331 Version 11.0.0, which can be slow, and at least SCells can be activated or deactivated using a Medium Access Control (MAC) control element. Since the activation process is based on MAC control elements—which are much faster than RRC signaling—an activation/de-activation process can quickly adjust the number of activated cells to match the number that are required to fulfill data rate needed at any given time. Activation therefore provides the possibility to keep multiple cells configured for activation on an as-needed basis.
In order to preserve the orthogonality in Uplink (UL), the UL transmissions from multiple UEs need to arrive time aligned at the eNodeB. This means the transmit timing of the UEs, which are under the control of the same eNodeB, should be adjusted to ensure that their received signals arrive at the eNodeB receiver at the same time which means more specifically arriving well within the cyclic prefix (CP). This ensures that the eNodeB receiver is able to use the same resources to receive and process the signals from multiple UEs.
In FIG. 5a an example is depicted in which two UEs (1 and 2) are located at different distances from the eNodeB. The UEs will need to initiate their UL transmissions at different times. UE2 which is far away from the eNodeB needs to start transmission earlier than UE1 which is located close to the eNodeB. This can for example be handled by time advance of the UL transmissions as depicted in FIG. 5b. The eNodeB sends a data frame to a UE1. UE1 receives this frame after a short delay because UE1 is located close to the eNodeB. UE1 has to send data in an UL frame a bit in advanced to ensure that the UL data is received at the eNodeB in time. UE1 starts its UL transmission before a nominal time given by the timing of the DL signal received by the UE1. UE2 is far away from the base station eNodeB. Therefore the DL data frame is received later and UE2 has to send the UL data frame much earlier than UE1 to make sure that its UL data frame is received by the eNodeB in time. The time between the sending of the UL data frame and the end of the reception of the DL data frame is called UL timing advanced. The UL timing advance is maintained by the eNodeB through timing alignment commands to the UE based on measurements on UL transmissions from that UE.
Through timing alignment commands, the UE is ordered to start its UL transmissions earlier or later. This applies to all UL transmissions except for random access preamble transmissions on Physical Random Access Control Channel (PRACH), i.e. including transmissions on the Physical Uplink Shared Channel (PUSCH), the Physical Uplink Control Channel (PUCCH), and the Sounding Reference Signals (SRS).
There is a strict relation between DL transmissions and the corresponding UL transmission. Examples of this are                the timing between a DL-Shared Channel transmission on PDSCH to the HARQ ACK/NACK feedback transmitted in UL (either on PUCCH or PUSCH);        the timing between an UL grant transmission on PDCCH to the UL-Shared Channel transmission on PUSCH.        
By increasing the timing advance value for a UE, the UE processing time between the DL transmission and the corresponding UL transmission decreases. For this reason, an upper limit on the maximum timing advance has been defined by 3GPP in order to set a lower limit on the processing time available for a UE. For LTE, this value has been set to roughly 667 us which corresponds to a cell range of 100 km (note that the TA value compensates for the round trip delay). In LTE Rel-10 there is only a single timing advance value per UE and all UL cells are assumed to have the same transmission timing. The reference point for the timing advance is the receive timing of the primary DL cell.
In LTE Release 11 a support for multiple timing advance values was introduced. The reason for this is that it would be possible to support aggregation of cells which have different UL reception points or cells which are not time aligned, e.g. the DL transmission timing is greater than a certain threshold.
To reduce the amount of signalling or to reduce the amount of processing in the eNodeB and the UE the concept of timing advance groups, TA groups (or TAGs), was introduced. The current assumption in 3GPP is that the serving cells sharing the same TA value (for example depending on the deployment) will be configured by the NW to belong to the same TA group.
Each TA group has an associated TA value, a TA timer and a timing reference which are common for all cells belonging to the TA group. When a TA group's TA timer is running the TA value is considered valid and all cells belonging to the TA group are considered time aligned and UL transmissions are allowed to the belonging serving cells. When the TA timer expires the cells are considered non-time aligned, or out of synch, and aside from network ordered preamble transmissions all other UL transmissions are not allowed to the belonging serving cells. For each TA group the UE selects one serving cell within the TA group which serves as the timing reference cell for all cells in that TA group. The DL reception timing of the timing reference cell is the timing reference for that TA group.
To change which TA group a serving cell shall belong to it has been agreed that RRC signalling shall be used. A cell which shall move to another TA group the network will de-configure the cell and configure the cell again, with the updated TA group index.
When a UE is configured with a cell it may need to re-tune the radio frontend to cover the spectrum of the configured cell. Similarly, when a serving cell is de-configured the UE may need to re-tune its radio frontend so as to not cover the de-configured cell. As a consequence of radio frontend re-tuning the UE may need to perform an interruption, or glitch, during which the UE is not able to receive and/or transmit signals on that radio frontend from and/or to the eNodeB. When a cell is activated or deactivated the UE may also perform a glitch, similar to the case of configuration or de-configuration. As a result a UE will create unnecessary interruptions which reduce the user experience. To change the TA group to which a serving cell belongs to, the eNodeB should de-configure the serving cell and configure it again. Whenever a serving cell is configured or de-configured the UE may create a glitch during which the UE is not accessible. If the UE is ordered to activate and deactivate a serving cell within a short period of time, which for example could be done to trigger a Power Headroom Report (PHR), the UE may also create a glitch.
Whenever a glitch is performed the network and UE will not be able to communicate hence reducing UE throughput and scheduling opportunities will be lost reducing system performance which will in the end negatively affect user experience.
The problem in particular occurs in intra-band contiguous Carrier Aggregation since in this case the UE typically has common RF components for more than one Component Carrier or Cell, e.g. 40 MHz RF BW for 2×20 Component Carriers. However the invention is also applicable to inter-band Carrier Aggregation and intra-band non-contiguous Carrier Aggregation in case the UE is capable of a common set of RF components which can receive and/or transmit more than one Component Carrier, i.e. if single radio chain can receive and/or transmit two or more Component Carriers.